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Date: Fri, 7 Dec 2018 20:30:03 +0000 From: Valentin Schneider <valentin.schneider@....com> To: Steve Sistare <steven.sistare@...cle.com>, mingo@...hat.com, peterz@...radead.org Cc: subhra.mazumdar@...cle.com, dhaval.giani@...cle.com, daniel.m.jordan@...cle.com, pavel.tatashin@...rosoft.com, matt@...eblueprint.co.uk, umgwanakikbuti@...il.com, riel@...hat.com, jbacik@...com, juri.lelli@...hat.com, vincent.guittot@...aro.org, quentin.perret@....com, linux-kernel@...r.kernel.org Subject: Re: [PATCH v4 00/10] steal tasks to improve CPU utilization Hi Steve, On 06/12/2018 21:28, Steve Sistare wrote: > When a CPU has no more CFS tasks to run, and idle_balance() fails to > find a task, then attempt to steal a task from an overloaded CPU in the > same LLC. Maintain and use a bitmap of overloaded CPUs to efficiently > identify candidates. To minimize search time, steal the first migratable > task that is found when the bitmap is traversed. For fairness, search > for migratable tasks on an overloaded CPU in order of next to run. > > This simple stealing yields a higher CPU utilization than idle_balance() > alone, because the search is cheap, so it may be called every time the CPU > is about to go idle. idle_balance() does more work because it searches > widely for the busiest queue, so to limit its CPU consumption, it declines > to search if the system is too busy. Simple stealing does not offload the > globally busiest queue, but it is much better than running nothing at all. > > The bitmap of overloaded CPUs is a new type of sparse bitmap, designed to > reduce cache contention vs the usual bitmap when many threads concurrently > set, clear, and visit elements. > > Patch 1 defines the sparsemask type and its operations. > > Patches 2, 3, and 4 implement the bitmap of overloaded CPUs. > > Patches 5 and 6 refactor existing code for a cleaner merge of later > patches. > > Patches 7 and 8 implement task stealing using the overloaded CPUs bitmap. > > Patch 9 disables stealing on systems with more than 2 NUMA nodes for the > time being because of performance regressions that are not due to stealing > per-se. See the patch description for details. > > Patch 10 adds schedstats for comparing the new behavior to the old, and > provided as a convenience for developers only, not for integration. > [...] I've run my usual tests ([1]) on my HiKey960 with - Just stealing (only misfit tests) - Stealing rebased on top of EAS (misfit + EAS tests), and with stealing gated by: ----->8----- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 17ab4db..8b5172f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7152,7 +7152,8 @@ done: __maybe_unused; rq_idle_stamp_update(rq); new_tasks = idle_balance(rq, rf); - if (new_tasks == 0) + if (new_tasks == 0 && + (!static_key_unlikely(&sched_energy_present) || READ_ONCE(rq->rd->overutilized)) new_tasks = try_steal(rq, rf); if (new_tasks) -----8<----- It all looks good from my end - if things were to go wrong on big.LITTLE platforms it'd be here. It might be a convoluted way of using this tag, but you can have my Tested-by: Valentin Schneider <valentin.schneider@....com> as a "it doesn't break my stuff" seal. As far as the patches go, with my last comments in mind it looks good to me so you can also have: Reviewed-by: Valentin Schneider <valentin.schneider@....com> for patches [2-8]. I haven't delved on the sparsemask details. As for patch 9, you might want to run other benchmarks (Peter suggested specjbb) to see if it is truly need. [1]: https://github.com/ARM-software/lisa/tree/next/lisa/tests/kernel/scheduler
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